CI: switch to Ubuntu 20.04 and don't use Docker image as build environment

Note that ubuntu-20.04 is preview currently, as stated in https://help.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners.

.github/workflows/docker_image.yml

@@ -14,7 +14,7 @@ on:
 
 jobs:
   update:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Clone repository
       uses: actions/checkout@v2

.github/workflows/packages.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       ANDROID_HOME: "/opt/termux/android-sdk"
       NDK: "/opt/termux/android-ndk"
@@ -24,12 +24,6 @@ jobs:
       uses: actions/checkout@v2
       with:
         fetch-depth: 1000
-    - name: Free additional disk space on host
-      run: |
-        sudo apt purge -yq $(dpkg -l | grep '^ii' | awk '{ print $2 }' | grep -P '(cabal-|dotnet-|ghc-|libmono|php)') \
-          java-common liblldb-6.0 libllvm6.0:amd64 liblxc-common libx11-6:amd64 mono-runtime-common monodoc-manual \
-          powershell ruby
-        sudo apt autoremove -yq
     - name: Build
       run: |
         BASE_COMMIT=$(jq --raw-output .pull_request.base.sha "$GITHUB_EVENT_PATH")
@@ -60,14 +54,8 @@ jobs:
           echo "[!] Force exiting as tag '%ci:no-build' was applied to HEAD commit message."
           exit 0
         fi
-        # Build local Docker image if setup scripts were changed.
-        # Useful for pull requests submitting changes for both build environment and packages.
-        if grep -qP '^scripts/(Dockerfile|setup-ubuntu\.sh)$' <<< "$CHANGED_FILES"; then
-          echo "Detected changes for environment setup scripts. Building custom Docker image now."
-          cd ./scripts
-          docker build -t termux/package-builder:latest .
-          cd ..
-        fi
+        # Install build tools
+        ./scripts/setup-ubuntu.sh
         # Remove duplicates and deleted packages from $PACKAGE_NAMES.
         # Write names of deleted packages to a list that will be passed to upload job for
         # further processing.

.github/workflows/repology_metadata.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   update:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Clone repository
       uses: actions/checkout@v2

.github/workflows/repository_cleanup.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   repo-maintenance:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Clone repository
       uses: actions/checkout@v2